In recent years, for display devices such as a liquid crystal display device, a light emitting display device, and an electrophoretic display device, while there is a strong demand for high pixel density, there is also a strong demand for reduction in power consumption. Under such circumstances, an active matrix type display device has been proposed in place of a simple matrix type display device of the related art.
That is, in the active matrix type display device, since a switching element is provided for each display pixel (or one dot), even if pixel density increases, it can be driven at a low voltage, which is preferable.
For a switching element of such an active matrix type display device, for example, a semiconductor device in which a circuit including a thin film transistor (hereinafter referred to as a “TFT”) using an oxide semiconductor in a channel region is formed on a substrate having light transmission is used (for example, Patent Literature 1 to 3).
As such an oxide semiconductor of the TFT, for example, a zinc oxide (ZnO) or In—Ga—Zn—O oxide semiconductor represented by a general formula of InGaO3(ZnO)m is used. Since the TFT using such oxide semiconductors in a channel region can obtain a higher field effect mobility than a conventional transistor using amorphous silicon, an operation speed is high, and it is preferable as a switching element of an active matrix type display device.
Specifically, in a conventional transistor using amorphous silicon, the field effect mobility remains at generally about 0.5 cm2/Vs. However, in the TFT using an oxide semiconductor, a higher field effect mobility of 10 to 20 cm2/Vs is obtained.
In addition, since it is possible to form an active layer by a sputtering method, an oxide semiconductor can be more easily formed than LTPS (low temperature poly-silicon crystal) or amorphous silicon.
Therefore, a semiconductor device in which a TFT using an oxide semiconductor in a channel region is formed on a substrate is expected to be applied to a wide range of fields of liquid crystal display elements, organic EL display elements, electronic paper, other electronic devices, optical devices, and the like.